Centralize domain profiles and isolate settings persistence

[KeyffMS]

Scope

Foundation refactor for milestone 0.5.0.50.

Changes

• introduce one canonical built-in visual-profile and transform registry;
• derive default profiles, built-in recognition, canonicalization and fallback validation from that registry;
• separate persisted JSON DTOs and legacy migration from runtime domain models;
• rename the executable mapping concept from ApplicationProfile to ApplicationAssignment across domain, UI and tests;
• change application counters and empty-state wording to assignment/application terminology;
• refactor settings normalization into explicit, ordered, idempotent passes;
• keep persisted JSON property names and schema compatibility intact;
• add catalog, persistence-boundary and migration regression tests;
• update architecture documentation.

Audit issues

Implements #50, #51, #59 and #68.

Validation

• Windows Release restore and build: passed;
• complete MSTest suite: passed;
• assignment terminology finalization: passed;
• temporary validation workflow removed from the final branch state;
• git diff --check: passed.

The repository's ordinary workflow is marked action_required without jobs because of repository-level workflow approval policy; the same head was validated by the guarded same-repository workflow.